A number of organic compounds have conventionally been known as photoconductive substances for electrophotographic photosensitive materials, and some of them have been confirmed to have considerably high photosensitivity.
Organic photoconductive substances exhibit superiority in many performance properties over inorganic ones, and contribute to broadening of applied techniques in the field of electrophotography. For example, use of organic photoconductive substances has first made it possible to produce transparent light-sensitive films, flexible light-sensitive films, and light-sensitive films that are light and easy to handle. Further, organic photoconductive substances possess various characteristics that cannot be expected from inorganic photoconductive substances, such as film-forming property and surface smoothness in the production of photoreceptors, as well as selectivity of charge polarity when applied to electrophotographic copying processes, and the like. In spite of these excellent characteristics in many respects, the organic photoconductive substances have not yet made a sufficient contribution to the technical field of electrophotography, due mainly to insufficient photosensitivity thereof.
The chief study of organic photoconductive substances has been from beginning directed to compounds such as those having complicated structures, various high-molecular aromatic or heterocyclic compounds, and so on. As a result, some compounds having considerably high sensitivities have been developed. In recent years, the center of the study tends to be shifted to methods of sensitization because of organic photoconductive compound having higher sensitivity than any other compounds so far known does not have high sensitivity enough to be practically used as such without being subjected to sensitization. Therefore, in practical use of the organic photoconductive substances, it is essential to choose the most effective method of sensitization to be applied.
Commonly known methods of sensitization include addition of sensitizing dyes and addition of Lewis acids, which can be applied to almost all of the organic photoconductive substances. The former method brings about sensitization by addition of spectral absorption characteristics of the dye to the organic photoconductive substance, and the latter method achieves sensitization through manifestation of new spectral sensitivity due to formation of a complex between the organic photoconductive substance and a donor acceptor (C-T band).
However, photosensitivity obtained by these conventional sensitization methods is still insufficient. Besides, the thus sensitized electrophotographic photoreceptors often undergo serious reduction in electrophotographic performance properties, such as photosensitivity, depending on conditions of preservation. In particular, when photoreceptors sensitized with dyes are preserved under high temperature and high humidity conditions, the sensitizing dyes decompose (discoloration) to reduce their sensitizing capability, which results in reduction of photosensitivity of the photoreceptors. The above-mentioned problem on preservation stability results in a serious hindrance to practical application of organic photoconductive substances to electrophotographic photosensitive materials.
In the light of the above-described circumstances, the inventors have developed electrophotographic photoreceptors sensitized with styryl dyes as disclosed in Japanese Patent Application (OPI) Nos. 164588/84 and 163047/85 (the term "OPI" as used herein means "unexamined published patent application"). Although these styryl dyes have succeeded to broaden the photosensitive wavelength regions and to improve photostability of photoreceptors, the problem of long-term stability of the photoreceptors has not yet come to a satisfactory solution.